Indicator dilution techniques have been used for well over a hundred years to determine various hemodynamic parameter, in particular blood flow volume etc. Indicator dilution involves the injection of some type of indicator into the circulatory system of the patient as part of the process of using the technique. For example two common types of indicators that can be injected into a circulatory system are saline solutions and dye solutions. Flow is determined by the concentration of the indicator after the indicator has flown through the system being studied. It should be noted that those skilled in the art are familiar with many other types of indicators including thermal dilution and many different techniques and sensors for implementing indicator dilution measurements.
Quite often indicator dilution is used with an extracorporeal circulating line which is attached to a patient to determine blood flow, cardiac output, blood water content or other hemodynamic parameters of the patient. An example of an extracorporeal line established in a patient for determination of cardiac output and other hemodynamic parameters is disclosed in pending U.S. patent application Ser. No. 11/370,721 filed Mar. 7, 2006 and titled System and Method for Determining Cardiac Blood Flow, which application is incorporated herein by reference as if set forth herein at length. Briefly, this patent application discloses an extracorporeal circuit that connects to the ends of a standard patient arterial catheter and venous catheter to allow the circulation of blood from the patient through the extracorporeal circuit and back into the patient. The purpose as described in detail in that patent application is to measure cardiac output and other blood parameters with indicator dilution techniques.
The use of indicator dilution techniques with extracorporeal circuits, in particular the bolus or sudden injection method of indicator dilution, can provide highly accurate readings of blood flow volume, cardiac output, etc. However, the sudden injection of the bolus of indicator creates problems and in fact can cause havoc to the flow of blood in the extracorporeal circuit. The sudden injection into an extracorporeal blood circuit of a bolus of indicator, such as a saline solution, tends to disrupt the ordinary flow of blood in the extracorporeal blood circuit in that portion of the circuit upstream from the site of injection. Typically, the indicator is being injected into an extracorporeal blood line that is already carrying its full capacity of blood flow. The sudden injection thus causes a spike in the pressure in the extracorporeal blood circuit at the point of injection as it is being forced into the line.
This sudden spike in pressure and interruption in flow of blood in the extracorporeal circuit typically results in a stoppage the flow of blood in the extracorporeal circuit upstream from the site of injection and quite often can cause a reversal of the flow of blood. This sudden stoppage or reversal of flow, even though it is only for the period of injection, creates problems with the final readings obtained by the indicator dilution technique. These problems can arise in passive systems where there is no pump or other device to regulate flow. These problems are also significant problems in systems where the blood flow in the extracorporeal circuit has been induced by and is being regulated by a pump. The stoppage of flow and/or its reversal upstream from the site of injection causes a corresponding slowing down or stopping of a pump located upstream from the injection site. The slowing or stopping of the pump introduces inaccuracies into the readings ultimately obtained with the indicator dilution injection.
Thus what is needed is a system and method that allows for the injection of a bolus into an extracorporeal circuit that prevents the sudden spike in pressure in the extracorporeal circuit from disrupting the flow of blood in the extracorporeal circuit. A system and method that eliminates the possibility that blood flow in the extracorporeal circuit above the site of injection will stop or reverse. A system and method that thus prevents conditions from developing in the extracorporeal circuit that interfere with a pump regulating flow of blood in the extracorporeal circuit.